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CRYSTAL RADIO RECEIVER
Definition
A Crystal radio receiver is a very simple radio receiver, popular in the early days
of radio. It needs no battery or power source and runs on the power received from
radio waves by a long wire antenna. It gets its name from its most important
component, known as a crystal detector, originally made with a piece of crystalline
mineral such as galena. This component is now called a diode.
Basics
The crystal radio receiver (also known as a crystal set) is a very simple kind of
radio receiver. It needs no battery or power source except the power received from
radio waves by a long outdoor wire antenna.
Introduction
Simple crystal radios are often made with a few hand made parts, like an antenna
wire, tuning coil of copper wire, crystal detector and earphones.
Because crystal radios are passive radio receivers, they are technically distinct
from ordinary radios containing active powered amplifiers in many respects. This
is because they must receive and preserve as much electrical power as possible
from the antenna and convert it to sound power whereas ordinary radios amplify
the weak electrical energy "signal" from the radio wave. Today making and
operating crystal radios is a popular hobby for many reasons, including:

Historical and nostalgic significance

The astonishing results one can get from its utter simplicity

The challenge of receiving weak distant signals without amplification
Crystal radios can be designed to receive almost any radio frequency since there is
no fundamental limit on the frequencies they will receive. The most common
crystal radios are designed for the AM Broadcast Band and the 49meter international short wave band, partly because the radio waves are stronger in
those bands. Early radios commonly received spark signals as low as 20 kHz and
below. Although crystal radios are designed to detect AM, they also frequently
detect FM fairly well which is in the 100 MHz range.
Groups of enthusiasts and a number of web sites are devoted to their construction.
Regular contests are held comparing the performance of various designs with each
other. Reportedly, modern diodes, ultra-thin litz wire inductors, and low loss
capacitors yield performance far beyond that of the original receivers.
How it works (simple version)
A crystal radio receives programs broadcast from radio stations. Radio stations
convert sound into radio waves and send out the waves everywhere. Radio waves
travel across the crystal radio antenna all the time. Radio waves make radio wave
electricity flow between the antenna wire and the ground wire. This electricity is
connected to the crystal radio by the antenna and ground wire. The crystal radio
uses a tuner to tune the electricity to receive just one station. The tuner can be as
simple as an adjustable one-slider tuning coil that resonates with the antenna
because the antenna also acts like a capacitor. Then it uses a crystal detector to
convert this radio wave electricity back to sound electricity. The detector can be
made from a special rock of galena in a holder. It uses earphones to convert the
sound electricity to sound you can hear in the earphones.
History
Crystal radio was invented by a long, partly obscure chain of discoveries in the late
1800s that gradually evolved into more and more practical radio receivers in the
early 1900s; and constitutes the origin of the field of electronics. The earliest
practical use of crystal radio was to receive dot and dash coded radio signals
transmitted by early amateur radio experimenters using very powerful spark-gap
transmitters. As electronics evolved, the ability to send voice signals by radio
caused a technological explosion in the years around 1920 that evolved into today's
radio broadcasting industry.
Early years
Early radio telegraphy used spark gap and arc transmitters as well as highfrequency alternators running at radio frequencies. At first a primitive detector
called a Branley Coherer was used to indicate the presence (or absence) of a radio
signal. However, these lacked the sensitivity to convert weak signals.
Greenleaf Whittier Pickard's U.S. Patent 836,531 "Means for receiving intelligence communicated by
electric waves" diagram.
Around 1906, researchers discovered that certain metallic minerals, such as galena,
could be used to detect signals. These devices were called 'crystal detectors'.
Greenleaf Whittier Pickard on August 30, 1906 filed a patent for a silicon crystal
detector, which was granted on November 20, 1906. Pickard's detector was
revolutionary in that he found that a fine pointed wire known as a "cat's whisker",
in delicate contact with a mineral produced the best semiconductor effect. A crystal
detector includes a crystal, a special thin wire that contacts the crystal and the stand
that holds the components in place. The most common crystal used is a small piece
of galena. Several other minerals also performed well as detectors. Another benefit
of crystals was that they could demodulate amplitude modulated signals. This
mode was used in radiotelephones and to broadcast voice and music for a public
audience.
Crystal sets represented an inexpensive and technologically simple method of
receiving these signals at a time when the embryonic radio broadcasting industry
was beginning to grow.
In 1922 the (then named) U.S. Bureau of Standards released a publication
entitled, Construction and Operation of a Simple Homemade Radio Receiving
Outfit. This article showed how almost any family having a member handy with
simple tools could make a radio and tune in to weather, crop prices, time, news and
the opera. More than any other system, the design contain therein, was responsible
for bringing radio to the general public. NBS followed that with more selective two
circuit version Construction and Operation of a Two-Circuit Radio Receiving
Equipment With Crystal Detector that was published the same year and is still
frequently built by enthusiasts today.
1920s and 1930s
U.S. Patent 1,748,435 , "Crystal radio apparatus", 1930. H. Adam
While there were a number of earlier experiments with radio broadcasts to the
general public, some historians consider the Autumn of 1920 to be the beginning
of radio broadcasting for entertainment purposes. Pittsburgh, PA, station KDKA,
owned by Westinghouse, received its license from the U.S. Department of
Commerce just in time to broadcast the Harding-Cox presidential election returns.
In addition to reporting on special events, broadcasts to farmers of crop price
reports were an important public service, in the early days of radio.
In 1921, factory-made radios were very expensive. Many of them cost more than
$2,000 USD (in year 2005 equivalent dollars), and less affluent families could not
afford to have one. Newspapers and magazines in many countries urged readers
interested in radio to acquire one of the inexpensive crystal sets or build their own.
To minimize the cost, many of the plans suggested winding the tuning coil on an
empty cylindrical oatmeal box. For years afterwards, home experimenters used
oatmeal boxes as coil forms for homemade radios. Even the crystal itself could be
made by mixing powdered sulfur into molten lead to form the lead sulfide
"crystal".
The crystal radio did not require batteries, but it did require the user to purchase a
commercially made set of headphones (or telephone receivers as they were called
in those days), since that accessory was not suitable for home construction.
Non-electric amplification

As gas lighting and kerosene lamps were widely used before the adoption of
electric power, their flame was used for sound amplification. A ceramic cone
with a pinhole on its tip was inserted in the middle of the flame, and an
earphone unit was attached to the cone's open bottom and sealed air-tight.
This acted like a little pump, modulating the fire by periodically sucking
away the combustible mixture at negative half-wave, and injecting it back on
positive.

Air pump amplification was first used in pathephones, where a pump was
driven by the same spring motor as a turntable. A needle-sized air pipe was
placed near a sound membrane, which acted as an air valve and modulated
the air flow, amplifying the sound. This method was easiliy converted for
crystal radio, either in a dedicated device or just by putting a "pumped"
pathephone's needle on an earpiece's membrane instead of a gramophone
record.
Valveless amplifier
"Carbon amplifier" consisting of a carbon microphone and an electromagnetic
earpiece sharing a common membrane and case. This was used in the telephone
industry and in hearing aids nearly since the invention of both components and
long before vacuum tubes. This could be readily bought or handcrafted from
surplus telephone parts for use with a crystal radio. Unlike vacuum tubes, it could
run with only a flashlight or car battery and had an almost infinite lifetime.
Cristadyne
In the early 1920s Russia, devastated by civil war, young scientist Oleg Losev was
experimenting with applying voltage biases to various kinds of crystals, with
purpose to refine the reception. The result was astonishing - with a zincyte (zinc
oxide) crystal he gained amplification. This was negative resistance phenomenon,
decades before the tunnel diode. After the first experiments, he built regenerative
and superheterodyne receivers, and even transmitters. However, this discovery was
not supported by authorities and soon forgotten and no device was produced in
mass quantity beyond a few examples for research. This was partly due to the low
education and overall ignorance of leadership, and partly due to the totalitarian
nature of the USSR regime.
The USSR opposed freedom of information, and registered all radio receivers until
1962, typewriters and copy machines until its demise, and the rule is not formally
denounced yet. Crystadine was produced in primitive conditions; it can be made in
a rural forge - unlike vacuum tubes and modern semiconductor devices. It is an
unwanted discovery to the authorities, and was consigned to obscurity. Oleg Losev
died 1943 in besieged Leningrad, abandoned and nearly forgotten.
1940s
When Allied troops were halted near Anzio, Italy during the spring of 1944,
personal portable radios were strictly prohibited, as the Germans had radio
detecting equipment that could detect the local oscillator signal of superheterodyne
receivers. Some resourceful GIs found that a crude crystal set could be made from
a coil made of salvaged wire, a rusty razor blade and a pencil lead for a diode. By
lightly touching the pencil lead to spots of blue on the blade, or to spots of rust,
they formed what is called a point contact diode and the rectified signal could be
heard on headphones or crystal ear pieces. The idea spread across the beachhead,
to other parts of the war, and to popular civilian culture. The sets were
dubbed "foxhole receivers" by the popular press, and they became part of the
folklore of World War II.
In some Nazi occupied countries there were widespread confiscations of radio sets
from the civilian population. This led to particularly determined listeners building
their own "clandestine receivers" which frequently amounted to little more than a
basic crystal set. However anyone doing so risked imprisonment or even death if
caught and in most parts of Europe the signals from the BBC (or other allied
stations) were not strong enough to be received on such a set. However there were
places such as the Channel Islands where it was possible.
Later years
While it never regained the popularity and general use that it enjoyed at its
beginnings, the circuit is still used. The Boy Scouts (who emerged as the unofficial
custodians of crystal radio lore) kept construction of a set in their program since
the 1920s. A large number of prefabricated novelty items and simple kits could be
found through the '50s and '60s, and many children with an interest in electronics
built one.
Building crystal radios was a craze in the 1920s, and again in the 1950s. Recently,
hobbyists have started designing and building sophisticated examples of the
instruments. As much effort goes into the visual appearance of these sets as well as
their performance, and some outstanding examples can be found.
Annual crystal radio DX contests and building contests allow these sets to compete
with each other and help form a community of interest in the subject.
Attempts at recovering RF carrier power
Crystal radio tuned to strong local station can be used just as a power source for
another amplified power-efficient (more often regenerative) receiver for distant
stations that can never be heard with plain crystal radio.
There is long history of less successful attempts and unverified claims to recover
the power in the carrier of the received signal itself. Traditional crystal sets use
half-wave rectifiers. As AM signals have a modulation factor of only 30% by
voltage at peaks, no more than 9% of received signal power (P = U2 / R) is actual
audio information, and 91% is just rectified DC voltage. Given that the audio
signal is unlikely to be at peak all the time, the ratio of energy is, in practice, even
greater. Considerable effort was made to convert this DC voltage into sound
energy. Some earlier attempts include a one-transistor amplifier in 1966.
Sometimes efforts to recover this power are confused with other efforts to produce
a more efficient detection.. This history continues now with designs as elaborate as
"inverted two-wave switching power unit" and bridge amplifiers.
Construction and operation
Importance of grounding
The long wire type antennas often used with crystal radios are Monopole antennas.
To receive signals from this type of antenna, a ground reference is needed to
provide a place for the antenna signal electricity to flow into and out of. Because
crystal radios have no other source of power than the electrical power they receive
from the antenna, the grounds for crystal radios must be much better than those
used by amplified radios. The importance of this is easy to overlook by those
familiar with amplified radios. Amplified radios use energy (voltage) detectors and
as such do not need to take much raw power from the antenna and need little or no
physical ground. Crystal radios rely on power detection and need to encourage as
much antenna current as possible to flow. This requires effective grounding.
The Naive Circuit
An impractical crystal radio circuit illustrated here is often naively proposed to
tune the medium wave AM broadcast band with a tuner made of a fixed parallel
coil and variable capacitor tank circuit with the antenna and ground connected
across it.
There are many practical crystal radio circuits, but connecting both the antenna and
a variable capacitor across a fixed coil like this makes tuning the whole two octave
AM Broadcast Band impractical.
The reason for this is technical. Most crystal radio antennas are about 60 feet long
and 20 feet high to be effective, and act something like a 250 to 300 pF capacitor
(actually antennas have capacitance, resistance and inductance, but these are
mostly capacitive.) If we connect a typical 250 pF antenna to the top of a tank that
uses a coil of more than about 75 μH we cannot tune above about 1400 kHz at all.
We must drop the size of the fixed coil below 75 μH to have any chance of tuning
the top of the band (around 1600 kHz or 1710 kHz.) Even with a 70 μH coil, we
need a 1000 pF variable capacitor to tune near the bottom of the band around 540
kHz. But now we need that same variable capacitor to vary down to about 4 pF to
reach the top of the band at 1710 kHz. That is a variation from 100 % to down to
about 0.4%. It is impractical to implement an air variable capacitor that can
achieve a variation range that even approaches that. In practice, stray capacitance
of air variable capacitors limits the range from 100 % to about 5%. Other kinds of
variable capacitors are seldom used for crystal radios because of their excessive
losses. Consequently, knowledgeable designers do not suggest this circuit.
Source: http://www.juliantrubin.com/encyclopedia/electronics/crystal_radio.html